Hereditary Methemoglobinemia

Although not usually diagnosed in the emergency department, patients may present with profound cyanosis that is related to a genetic abnormality. Hereditary methemoglobinemia results from either an enzymatic deficiency (i.e., NADH cytochrome-b5 reductase) or from the presence of an amino acid substitution within the hemoglobin molecule itself (i.e., Hb M). Patients with NADH cytochrome- b5 reductase deficiency develop methemoglobin levels of 20 to 40 percent, but these are easily reduced by the administration of daily oral doses of methylene blue or vitamin C. 4 Cyanosis in these individuals begins at birth, but they remain asymptomatic and develop normally. Hemoglobin M is an abnormal form of hemoglobin in which it's tertiary structure is altered and the heme iron exists in an environment favoring the ferric form. This disease only occurs in the heterozygous form, as the homozygous form is incompatible with life. Currently there is no treatment for this form of methemoglobin. As with NADH cytochrome-b5 reductase deficiency, patients develop profound cyanosis, but tolerate the elevated methemoglobin concentrations extremely well.

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